This invention relates to Fabry-Perot optical filters and in particular to the suppression of polarisation noise and crosstalk in Fabry-Perot wavelength tuning components.
To enhance the data carrying capacity of single mode optical fibre systems, wavelength multiplexing/demultiplexing techniques can be used. A specified wavelength can be separated from a multitude of channels at different wavelengths using wavelength selective components. One of a number of techniques is to position an interference filter (Fabry-Perot etalon) in front of each receiver. Each filter would pass only one wavelength out of all the wavelengths present in the system.
To obtain a large number of channels, the pass band of the filters would have to be very narrow. Typical channel spacings are 4 nm, implying that the filter is required to have a full width at half maximum of less than 1 nm. As indicated above, one method of producing suitable filters is to use Fabry-Perot etalons. The transmission wavelength of a Fabry-Perot etalon can be lowered by rotating the etalon (filter) away from normal incidence. This allows one filter to cover a range of wavelengths. The filter can either be pre-adjusted and set to a particular wavelength or can be used to actively "tune in" to any of the wavelengths available.
However, as a filter is rotated, the response of the filter varies, depending upon the state of polarisation of the incoming light. The two extremes of behaviour for a given angle of incidence .theta. of a light beam 2 to a filter 1 are for light polarised in the plane of incidence 3 and for light polarised normal to the plane of incidence 4 (FIG. 1a). The corresponding responses 3.sup.1 and 4.sup.1 of the filter are illustrated in FIG. 1B. The response of the filter to the two orthogonal polarisations will depend upon the absorption of the material used to make the filter. FIG. 2 shows the variation in insertion loss with peak transmission wavelength as the filter is rotated from 0.degree. to 30.degree. for a filter with no absorption (.alpha.=0/cm), whilst FIG. 3 shows the same curve for an absorption .alpha. of 15/cm. The results illustrated were obtained for a Fabry-Perot cavity of seven half wavelengths, each mirror being comprised of eleven layers.
The change in insertion loss as the filter is rotated is greater when absorption is present. As well as the insertion loss being different for the orthogonal polarisation, the peak transmission wavelengths differ. This has the effect of increasing the difference in transmission for the two orthogonal polarisations.
The difference in transmission at a particular wavelength is the polarisation noise. An example of this is illustrated in FIG. 4, where the difference in peak heights is less than the polarisation noise 5. FIG. 5 shows a graph of polarisation noise plotted against filter rotation angle for an absorption of 0/cm and 15/cm. A typical figure for the level of polarisation noise that can be tolerated by a system is 0.1 dB.
Another important factor when designing multiwavelength systems is the interchannel isolation or crosstalk. In order to obtain sufficient isolation between channels, it is necessary to have two filters in series. These factors imply that the level of polarisation noise that can be tolerated by each filter is 0.05 dB. As is apparent from FIG. 5, this restricts the tuning range of a non-absorbing filter to 14.degree., or 20 nm, whilst an absorbing filter is restricted to 5.degree., or 3 nm.
In our GB Patent Application GB 2 223 324 A, the two filters are set up so that they can be tuned by rotating them about orthogonal axes. The response is then independent of the polarisation. FIG. 6 shows this arrangement schematically. One filter 10 rotates about axis 11, whereas another filter 12 rotates about axis 13. FIG. 7 shows the response of such a filter with zero absorption. This filter can be tuned over 20 nm before polarisation noise limits the performance. With two orthogonal filters in series (curve 15), the insertion loss is constant over the first 20 nm and then starts to rise, i.e. polarisation noise is eliminated at the expense of insertion loss. The insertion loss is still only 0.7 dB when the filter has been tuned 50 nm away from the nominal wavelength. Therefore, the use of orthogonal filters has more than doubled the usable range of the filter. FIG. 8 shows the response (curve 16) of a similar filter which has an absorption of 15/cm. The tuning range of this basic two element filter was limited to 3 nm as indicated above, however, it is now possible to use this filter over 50 nm before the insertion loss begins to rise steeply. Thus in this case the use of orthogonal filters has increased the tuning range by almost a factor of 20.
An object of the present invention is to provide improved filter arrangements and in particular arrangements where it is not necessary separately to rotate two orthogonally mounted interference filters as was proposed in our prior GB patent application.